Digital multimedia capabilities can be incorporated into a wide range of devices, including digital televisions, digital direct broadcast systems, wireless communication devices, wireless broadcast systems, personal digital assistants (PDAs), laptop or desktop computers, digital cameras, digital recording devices, video gaming devices, video game consoles, cellular or satellite radio telephones, digital media players and the like. Digital multimedia devices may implement video coding techniques, such as MPEG-2, ITU-H.263, MPEG-4, or ITU-H.264/MPEG-4 Part 10, Advanced Video Coding (AVC), to transmit and receive or store and retrieve digital video data more efficiently. Video encoding techniques may perform video compression via spatial and temporal prediction to reduce or remove redundancy inherent in video sequences.
In video encoding, the compression often includes spatial prediction, motion estimation and motion compensation. Intra-coding relies on spatial prediction and transform coding, such as discrete cosine transform (DCT), to reduce or remove spatial redundancy between video blocks within a given video frame. Inter-coding relies on temporal prediction and transform coding to reduce or remove temporal redundancy between video blocks of successive video frames of a video sequence. Intra-coded frames (“I-frames”) are often used as random access points as well as references for the inter-coding of other frames. I-frames, however, typically exhibit less compression than other frames. The term I-units may refer to I-frames, I-slices or other independently decodable portions of an I-frame.
For inter-coding, a video encoder performs motion estimation to track the movement of matching video blocks between two or more adjacent frames or other coded units, such as slices of frames. Inter-coded frames may include predictive frames (“P-frames”), which may include blocks predicted from a previous frame, and bidirectional predictive frames (“B-frames”), which may include blocks predicted from a previous frame and a subsequent frame of a video sequence. The terms P-frames and B-frames are somewhat historic in the sense that early coding techniques limited prediction in specific directions. Some newer coding formats and standards do not limit the prediction direction of P-frames or B-frames
Consistent with newer video standards such as ITU H.264, for example, B-video blocks may be predicted from two different lists which do not necessarily need to be “bi-directional.” In other words, B-video blocks may be predicted from two lists of data, which may correspond to data from two previous frames, two subsequent frames, or one previous frame and one subsequent frame. In contrast, P-video blocks are predicted based on one list, i.e., one data structure, which may correspond to one predictive frame, e.g., one previous frame or one subsequent frame. B-frames and P-frames may be more generally referred to as P-units and B-units. P-units and B-units may also be realized in smaller coded units, such as slices of frames or portions of frames. B-units may include B-video blocks, P-video blocks or I-video blocks. P-units may include P-video blocks or I-video blocks. I-units may include only I-video blocks.
For P- and B-video blocks, motion estimation generates motion vectors, which indicate the displacement of the video blocks relative to corresponding prediction video blocks in predictive frame(s) or other coded units. Motion compensation uses the motion vectors to generate prediction video blocks from the predictive frame(s) or other coded units. After motion compensation, a residual video block is formed by subtracting the prediction video block from the original video block to be coded. The video encoder usually applies transform, quantization and entropy coding processes to further reduce the bit rate associated with communication of the residual block. I- and P-units are commonly used to define reference blocks for the inter-coding of P- and B-units.
Transcoding is generally a process of converting video content from one encoded format to another encoded format. For example, video content may be transcoded from an ITU-H.263 format to an ITU-H.264 format or from the ITU-H.264 format to the ITU-H.263 format. Transcoding is commonly used to allow a broadcast device or other device, to effectively communicate video content encoded in one format to a device that supports another format. Unfortunately, conventional transcoding is often very computationally intensive. For example, the encoded video content may need to be decoded from the first format and then re-encoded in the second format, which may require extensive amounts of video processing for every frame of a video sequence.